[load_stac] allow load from static Collection

[clausmichele]

Allow to use load_stac with a static STAC Collection. It will be necessary for the test suite, since we plan to host the STAC Collection on GitHub (no API).

You can test it with the one I prepared:

from openeo.local import LocalConnection
local_conn = LocalConnection("./")

url = "https://raw.githubusercontent.com/clausmichele/openeo-test-suite/eurac_process_graphs/src/openeo_test_suite/data/SENTINEL2_L2A_SAMPLE/SENTINEL2_L2A_SAMPLE.json"
bands = ["B04"]

s2_cube = local_conn.load_stac(url=url,
   bands=bands,
)
s2_cube.execute()

[codecov]

Codecov Report

Attention: 22 lines in your changes are missing coverage. Please review.

Comparison is base (8d9e3fb) 79.42% compared to head (3ed5d80) 78.38%.

Files	Patch %	Lines
...ocesses_dask/process_implementations/cubes/load.py	8.33%	22 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #203      +/-   ##
==========================================
- Coverage   79.42%   78.38%   -1.04%     
==========================================
  Files          30       30              
  Lines        1468     1490      +22     
==========================================
+ Hits         1166     1168       +2     
- Misses        302      322      +20     

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[clausmichele]

I need to clarify if the self-contained STAC Collection is actually valid (currently a valid STAC but I can't find any other examples doing something like this), I will re open the PR once clarified.